Axleboxes mounted on railway vehicles, such as trains, passenger coaches and freight cars, are the linking design elements between a rotating wheelset and the quasi-static frame of a bogie. An axlebox comprises a housing receiving a bearing unit.
Furthermore, it is known to provide an axlebox with a device for backing the bearing unit mounted on the axle. Typically, the backing device has an annular shape and is mounted around the axle, between the bearing unit and an abutment formed on the axle. The backing device performs a mechanical connection between the axle and the bearing unit. The abutment is designed with a vertical face and a round shape in connection with the diameter of the bearing unit, in order to avoid stress concentration. The backing device is designed to equally distribute the pressure coming from the bearing unit and axial forces to the abutment. To avoid micro displacement and fretting, the backing device is generally press fitted on the axle. Preferably, the backing device associated with the axlebox housing or a rear cover has a special shape for ensuring sealing between the bearing unit and the external environment of the axlebox, in order to avoid grease contamination.
In practice, the abutment must be designed with a sufficient mechanical strength to press the bearing unit with a suitable mounting force when the axlebox is mounted on the axle, then to support axial forces coming from curve and switch points when the vehicle is in service. However, an oversized diameter of the abutment would negatively impact the size of the equipment mounted between the two axleboxes. Indeed, the diameter of the equipment, including wheels, gearbox and brakes, must be larger than the centering diameter of the backing device to avoid mechanical interference during assembly.
Thus, the diameter of the backing device is a compromise between the diameter of the bearing unit and the diameter of the equipment. It often results in an optimized design with potential weaknesses, namely surface deformation on the contact interface between the backing device and the axle, stress concentration on certain areas of the axle, insufficient mounting force of the bearing unit due to the deformation risk. Besides, on some axleboxes, equipment and wheels are oversized to allow their mounting on the axle.
U.S. Pat. No. 6,561,559 discloses several examples of backing devices, adapted to equip an axlebox comprising a bearing unit mounted on an axle.